I only found this out when I went to tax and change class to disabled on second car, it was refused and I had to call and get a new one sent out, after discussing this with the PIP rep on the phone they explained it to me and told me under no circumstances to allow the P.O. to stamp the form.
I can only assume that when it comes to yearly renewal you have to send a scanned/photographed copy of the form and anything on it will cause the invalidation.
You said you had trouble getting the form? Strange as both times I found it very easy and quick to arrive (2 days the second time)